1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a method of fabricating a polarizing plate for an LCD device.
2. Discussion of the Related Art
Until recently, display devices have typically used cathode-ray tubes (CRTs). More recently, considerable effort has been expended to research and develop thin film transistor liquid crystal display (TFT-LCD) devices having thin profiles, light-in-weight and have low power consumption as substitutes for CRTs.
Liquid crystal display (LCD) devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. The liquid crystal molecules have long, thin, shapes and have an initial alignment direction including initial pretilt angles. The alignment direction can be controlled by applying an electric field to influence the alignment of the liquid crystal molecules. Due to the optical anisotropy property of liquid crystal, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling the applied electric field, an image having a desired brightness can be produced.
The LCD devices include first and second substrates spaced apart and facing each other, and a liquid crystal layer interposed between the first and second substrates. Further, a polarizing plate is formed on an outer surface of each of the first and second substrates. A polarization component of light parallel to a transmission axis of the polarizing plate passes through the polarizing plate and the other polarization component of the light is shielded by the polarizing plate. As a result, transmittance of light is determined by the arrangement of the polarization axes of the two polarizing plates and the alignment of the liquid crystal layer.
The polarizing plate includes a polarizing layer and transparent films on both surfaces of the polarizing layer. The polarizing plate is formed by cutting a polarizing roll film into polarizing layers and forming first and second support layers of a cellulose material such as tri-acetate cellulose (TAC), an anti-glare layer and an adhesive layer on each of the polarizing layers. In addition, the polarizing roll film is formed by dyeing a raw film with one of iodine (I2) and dichroic dye.
FIG. 1 is a cross-sectional view showing a polarizing plate according to the related art. In FIG. 1, a polarizing plate 10 includes a polarizing layer 11, a first support layer 13a on one surface of the polarizing layer 10, a second support layer 13b on the other surface of the polarizing layer 11, an anti-glare layer 15 on the first support layer 13a and an adhesive layer 17 on the second support layer 13b. Although not shown in FIG. 1, a hard coating layer for protecting the polarizing plate 10 and a sticking prevention layer for preventing sticking to adjacent layers may be formed on the anti-glare layer 15.
The process of forming the polarizing layer 11 is the subject of significant research and development to obtain a uniform polarization property for a higher display quality. In the process of forming the polarizing layer 11, a raw film (not shown) of large molecules is dyed by dipping in a solution of iodine (I2) or dichroic dye, and the dyed raw film is stretched to arrange the iodine (I2) molecule or the dichroic dye molecule parallel to a stretch direction. For the stretch step, the raw film is wound on a roller (not shown) in a bath of the solution of iodine (I2) or dichroic dye.
FIG. 2 is a plan view showing a transmission axis of a polarizing roll film according to the related art. In FIG. 2, a polarizing roll film 20 has a plurality of polarizing plate areas 11a disposed in central (A), left (B) and right (C) regions. Since the polarizing roll film 20 is stretched and wound, transmission axes 22 of the polarizing roll film 20 are distorted and deformed due to a stress generated in the stretch process. As a result, the transmission axes 22 are not uniformly aligned along a stretch direction. Instead, the polarizing roll film 20 has a deviation in the direction of the transmission axes 22.
The polarizing roll film 20 is cut into polarizing plates 11 (of FIG. 1) along the plurality of polarizing plate areas 11a and polarizing plates 10 (of FIG. 1) are completed by forming the first support layer 13a (of FIG. 1), the anti-glare layer 15 (of FIG. 1), the second support layer 13b (of FIG. 1) and the adhesive layer 17 (of FIG. 1). The plurality of polarizing areas 11a are disposed such that two opposite sides of each polarizing area 11a are parallel to two opposite sides of the polarizing roll film 20. Since the transmission axes 22 of the polarizing roll film 20 are distorted, the polarizing plates 10 (of FIG. 1) have a deviation in the direction of the transmission axes 22. The deviation in the direction of the transmission axes 22 causes a color stain or a decoloration in an LCD device including the polarizing plate 10 (of FIG. 1). Further, the deviation in the direction of the transmission axes 22 causes deterioration of the polarization property, thereby reducing the contrast ratio of an LCD device including the polarizing plate 10 (of FIG. 1).
FIG. 3 is a histogram showing deviation in direction of transmission axes of polarizing plates according to the related art. In FIG. 3, transmission axes of polarizing plates are measured according to central, left and right regions A, B and C (of FIG. 2) of a polarizing roll film 20 (of FIG. 2). The x-axis of the histogram represents an angle between a transmission axis of a polarizing plate and a reference direction and the y-axis of the histogram represents the numbers of polarizing plates having the corresponding angle. The reference direction is determined as a lengthwise direction of the polarizing plates, which is the same as a lengthwise direction of the polarizing roll film 20. As shown in FIG. 3, the polarizing plates in the central region A have the transmission axes of an angle between about −0.075° to about −0.05°. In addition, the polarizing plates in the left region B have the transmission axes of an angle between about −0.3° to about −0.275°, and the polarizing plates in the right region C have the transmission axes of an angle of about 0.075°. As a result, the transmission axes of the polarizing plates have an angle between about −0.3 to about 0.075 with respect to the reference direction, and the polarizing plates have relatively great deviation, a range width of about 0.375°, in the direction of the transmission axes.
Recently, a polarizing plate using a discotic liquid crystal molecule has been researched and developed for improving viewing angle. FIG. 4 is a cross-sectional view showing a polarizing plate using a discotic liquid crystal molecule according to the related art. In FIG. 4, a polarizing plate 30 includes a polarizing layer 31, a first support layer 33a on one surface of the polarizing layer 30, a second support layer 33b on the other surface of the polarizing layer 31, an anti-glare layer 35 on the first support layer 33a and an adhesive layer 37 on the second support layer 33b. Although not shown in FIG. 4, a hard coating layer for protecting the polarizing plate 30 and a sticking prevention layer for preventing stick to adjacent layers may be further formed on the anti-glare layer 35.
The second support layer 33b includes discotic liquid crystal molecules 41 arranged in a hybrid type for improving viewing angle. The second support layer 33b of a tri-acetate cellulose (TAC) including discotic liquid crystal molecules 41 may be referred to as a WA (wide view)-TAC film and function as a retardation film.
The polarizing plate 30 is applied to an LCD device such that the adhesive layer 37 is attached to a liquid crystal panel (not shown). After the polarizing plate 30 is attached to a liquid crystal panel, the polarizing plate 30 may be heated by an environmental heat source such as a backlight unit. When the polarizing plate 30 is heated, the first and second support layers 33a and 33b expand because of heat. On the contrary, since a contractile force is accumulated in the polarizing layer 31 as a tension during the stretch step for the raw film, the polarizing layer 31 contracts with heat. Accordingly, when the polarizing plate 30 is heated, a stress is generated between the polarizing layer 31 and each of the first and second support layers 33a and 33b due to the difference in response to heat. The stress may deteriorate properties of the polarizing plate 30, thereby degrading the display quality of an LCD device.
Specifically, the stress between the polarizing layer 31 and the second support layer 33b may cause distortion in the arrangement of the discotic liquid crystal molecules 41. FIG. 5 is a cross-sectional view showing distortion in arrangement of discotic liquid crystal molecules in a polarizing plate according to the related art. In FIG. 5, when a heat is applied to a polarizing plate 30 (of FIG. 4), the arrangement of the discotic liquid crystal molecules 41 is distorted due to the stress generated by contraction of the polarizing layer 31 and expansion of the second support layer 33b. The degree of distortion in the arrangement of the discotic liquid crystal molecules 41 is proportional to the difference between the contractile force of the polarizing layer 31 and the expansive force of the second support layer 33b. Since the difference between contractile force of the polarizing layer 31 and expansive force of the second support layer 33b is maximized in edge portions of the polarizing plate 30, the distortion in the arrangement of the discotic liquid crystal molecules 41 is maximized in the edge portions. Accordingly, the polarizing plate 30 has a non-uniform polarization property throughout the whole display area, and degradation in the polarization property of the polarizing plate 30 is maximized in the edge portions of an LCD device.